Abstract: This paper presents the design and construction of a low cost under and over voltage protective device, which was fabricated using a microcontroller, transistor, IC and other discrete components. A microcontroller PIC16F877 is at the heart of the device which performs the major control of the device. The device is simple and of low cost. It can withstand loads up to 5 ampere and the required set voltage range for the device to allow supply to the connected load at the output varies from 170 270 Volts. This device can be used directly as standalone equipment between the mains supply and the load, or it may be inserted between an existing automatic/manual stabilizer and the load. The over/under voltage cut-off with time delay provides over/under-voltage protection, and protection against transients. The over/under voltage cut-off with ON-Time delay provides various types of protection 1) Over-voltage protection 2) Under-voltage protection 3) Protection against transients 4) Protection to load from frequency turning ON & OFF by providing time delay.

Keywords: Microcontroller, over voltage, under voltage, protection.